Sildenafil citrate (SC), known for its role as a phosphodiesterase type-5 (PDE5) inhibitor, enhances the activity of cyclic guanosine monophosphate (cGMP). This study explores SC’s influence on blood sugar regulation and blood-related parameters in rats with diabetes induced by streptozotocin (STZ). 50 male Wistar rats were randomly assigned to four experimental groups: (i) a control group (n = 10), (ii) a control group receiving SC (n = 10), (iii) a diabetic group (n = 15), and (iv) a diabetic group treated with SC (n = 15). Diabetes was triggered using a single intraperitoneal dose of STZ (50 mg/kg), followed by oral administration of SC at 20 mg/kg daily for six weeks. Blood analyses were conducted to evaluate fasting glucose, insulin, HbA1c, liver enzymes (AST, ALT), renal markers (urea, creatinine), and coagulation profiles (PT, aPTT, fibrinogen, protein C, protein S). Diabetic rats showed significant increases in glucose, HbA1c, AST, ALT, urea, creatinine, and fibrinogen levels, along with reductions in insulin, aPTT, protein C, and protein S compared with non-diabetic controls. PT remained unaffected. SC did not significantly alter any parameters in non-diabetic rats, but in diabetic ones, it restored most measurements toward normal levels (P < 0.05). These findings indicate that SC may support better glycemic control and improve microvascular function, offering potential therapeutic value in mitigating diabetes-related complications.
Diabetes mellitus (DM) is a persistent and widespread metabolic disorder that poses a major global health burden, impacting populations in both industrialized and developing nations. As of 2013, approximately 400 million individuals were living with DM, with projections suggesting this figure could reach 600 million by 2035 [1]. A hallmark of diabetes is prolonged high blood glucose (hyperglycemia), which facilitates non-enzymatic glycation reactions between glucose and biological molecules like proteins, lipids, and peptides. This process leads to the formation of advanced glycation end-products (AGEs), which are implicated in sustained inflammation and microvascular damage—particularly affecting organs such as the kidneys, eyes, and peripheral nerves [2].
Sildenafil citrate (SC), a well-established phosphodiesterase type-5 (PDE-5) inhibitor, is primarily known for its vasodilatory effects and is commonly used to manage erectile dysfunction [3]. Beyond this, it has been investigated for its therapeutic potential in various conditions such as lupus, pulmonary hypertension, and certain cardiovascular disorders [2, 4]. SC functions by inhibiting PDE-5, thereby increasing intracellular cyclic guanosine monophosphate (cGMP) levels. cGMP, in turn, enhances nitric oxide (NO) signaling, leading to vascular smooth muscle relaxation, improved blood flow, inhibition of platelet aggregation, and better microcirculatory function [5]. Emerging research also links NO to glucose and insulin regulation [6].
Both insulin and SC stimulate NO synthesis in metabolically active tissues such as the liver, skeletal muscle, and pancreatic beta cells, all of which are central to maintaining glucose balance [7, 8]. While the therapeutic use of SC in diabetic patients is becoming more common, there remains limited data on its long-term impact on glucose regulation and coagulation profiles. This study, therefore, investigates the effects of daily oral sildenafil (20 mg/kg) on glucose metabolism and coagulation disturbances in streptozotocin (STZ)-induced diabetic male rats.
Fifty adult male Wistar albino rats (25–30 g) were used in this study. They were sourced from the animal facility of the Faculty of Medicine at King Abdulaziz University (KAU), Jeddah, Saudi Arabia. Before the experiment, rats were housed under standard laboratory conditions—ambient temperature control, a 12-hour light/dark cycle—and given unrestricted access to water and standard pellet feed. The experimental protocol adhered to KAU’s institutional animal care guidelines and followed the International Ethical Standards for Laboratory Animal Use. Ethical approval was obtained from the Research Ethics Committee at KAU.
Diabetes was induced in designated groups with a single intraperitoneal injection of streptozotocin (STZ) at 50 mg/kg, dissolved in freshly prepared citrate buffer (0.1 M, pH = 4.5). The STZ was procured from Sigma-Aldrich (St. Louis, MO, USA). To mitigate acute hypoglycemia, rats were provided with a 10% glucose solution alongside their regular diet for 48 hours post-injection. Fasting blood glucose levels were measured 72 hours after injection using a glucometer. Rats with glucose levels ≥ 250 mg/dL were classified as diabetic.
SC (Viagra®, Pfizer, St. Louis, MO, USA) was crushed and mixed with 2–3 mL of water to form a uniform suspension. A dose of 20 mg/kg was administered orally once daily for six weeks using a gavage needle.
Rats were randomly divided into four experimental groups:
Group I (control): Received citrate buffer only (n = 10).
Group II (SC control): Received citrate buffer and SC treatment (n =10).
Group III (diabetic control): Received STZ injection only (n = 15).
Group IV (diabetic + SC): Received STZ injection and daily SC treatment (n = 15).
After six weeks of treatment, rats were fasted overnight (water permitted), and blood was drawn from the retro-orbital plexus under light ether anesthesia. Blood samples were centrifuged at 2500 rpm for 15 minutes to obtain serum for biochemical analyses.
Glycemic markers: Fasting blood glucose was measured colorimetrically using a VITROS 250 autoanalyzer. HbA1c was quantified via the VITROS 5,1 FS system.
Insulin: Measured using a chemiluminescent immunoassay (CLIA) on a LIAISON system.
Liver and kidney function tests: AST, ALT, urea, and creatinine were measured using standard colorimetric methods.
Coagulation tests: PT, aPTT, fibrinogen, protein C, and protein S levels were assessed with Biomed diagnostic kits and analyzed on an automated coagulation analyzer according to the manufacturer’s protocol.
Data were analyzed using SPSS software (version 21.0; SPSS Inc., Chicago, IL, USA). Results are expressed as mean ± standard error of the mean (SEM). One-way analysis of variance (ANOVA) was used, followed by Tukey’s post hoc test to assess group differences. A P-value < 0.05 was considered statistically significant.
The intraperitoneal administration of STZ (50 mg/kg BW) effectively induced hyperglycemia in all treated male rats, confirming successful diabetes induction. Importantly, no mortality was observed during the study period. Additionally, the two control subgroups—untreated and SC-treated—showed no statistically significant differences in any biochemical parameters, indicating that sildenafil citrate alone had no adverse or measurable metabolic effects in healthy animals.
Table 1 summarizes the effects of STZ and SC on serum glucose, insulin, and glycated hemoglobin (HbA1c) levels. As expected, the diabetic groups exhibited a substantial increase in fasting blood glucose levels compared with non-diabetic controls (P < 0.05), confirming the hyperglycemic state induced by STZ. However, diabetic rats receiving daily oral sildenafil citrate showed a marked reduction in glucose levels compared to their untreated diabetic counterparts (P < 0.05), suggesting a glycemia-lowering effect of SC.
Moreover, STZ-induced diabetes caused a significant suppression of serum insulin levels relative to the control group. Interestingly, insulin levels in SC-treated diabetic rats rose significantly (P < 0.05), approaching near-normal levels, suggesting a potential insulin-sensitizing or beta-cell-protective effect of SC.
Similarly, HbA1c—a key marker of long-term glycemic control—was notably elevated in the diabetic control group (6.91 ± 0.04%) compared to the non-diabetic control (4.20 ± 0.06%). Treatment with SC significantly mitigated this increase, bringing HbA1c down to 4.80 ± 0.03% (P < 0.05). This reinforces the notion that SC may improve not only acute glucose levels but also long-term metabolic outcomes in diabetic subjects.
Collectively, these findings demonstrate that sildenafil citrate improves glycemic control in diabetic rats. The improvements in both glucose and insulin profiles, as well as the normalization of HbA1c, suggest SC’s potential to counteract diabetic dysregulation. This effect may be mediated by enhanced nitric oxide signaling and improved microvascular function, both of which have been linked to insulin sensitivity and pancreatic function in prior studies.
Table 1. Serum glucose, insulin, and % HbA1c in the studied groups
Group I | Group II | Group III | Group IV | |
Glucose (mg/dl) | 73.21 ± 0.74 | 74.0 ± 0.63 | 265.5 ± 0.78* | 105.60 ± 0.72£※ |
Insulin (IU/dl) | 11.7 ± 1.2 | 12.0 ± 1.1 | 4.18 ± 0.3* | 9.2 ± 1.1£※ |
HbA1c % | 4.20 ± 0.06 | 4.23 ± 0.09 | 6.91 ± 0.04* | 4.80 ± 0.03£※ |
Data are expressed as mean ± SE, *: P < 0.05 as compared to the controls (I and II), £: P < 0.05 as compared to the diabetic rats’ III, ※: P < 0.05 as compared to the controls (I and II).
After six weeks, biochemical analysis revealed significant alterations in liver function markers among diabetic rats. Compared to the normal control groups (groups I and II), diabetic rats (group III) exhibited notably elevated levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT), with statistical significance (P < 0.05). However, treatment with sildenafil citrate (group IV) resulted in a meaningful reduction in these liver enzyme levels. Notably, there were no significant differences in AST and ALT values between the SC-treated and untreated control groups.
Renal function markers—serum creatinine and urea—were also significantly elevated in the diabetic group relative to controls (P < 0.05). Diabetic rats treated with sildenafil (group IV) demonstrated substantial reductions in creatinine and urea concentrations, suggesting a protective effect on renal function. Again, no significant differences were observed between groups I and II (Table 2).
Table 2. Effect of sildenafil on liver and kidney function parameters
Parameter | Group I (control) | Group II (control + SC) | Group III (diabetic) | Group IV (diabetic + SC) |
AST (U/L) | 94.7 ± 2.7 | 95.3 ± 2.2 | 179.6 ± 6.4 * | 114.8 ± 6.1 £ ※ |
ALT (U/L) | 24.63 ± 1.81 | 23.16 ± 0.89 | 57.63 ± 5.70 * | 32.67 ± 2.3 £ ※ |
Urea (mg/dL) | 28.20 ± 0.24 | 26.81 ± 0.17 | 93.85 ± 1.45 * | 56.9 ± 1.02 £ ※ |
Creatinine (mg/dL) | 0.56 ± 0.08 | 0.60 ± 0.01 | 2.42 ± 0.34 * | 0.93 ± 0.01 £ ※ |
Data expressed as mean ± SE. *: P < 0.05 vs. groups I and II; £: P < 0.05 vs. group III; ※: P < 0.05 vs. groups I and II.
The impact of SC on coagulation markers is outlined in Table 3. No significant differences were observed between the control groups in any coagulation-related measurements. Similarly, prothrombin time (PT) remained statistically unchanged across all groups, including diabetic rats.
However, the activated partial thromboplastin time (aPTT) was significantly reduced in diabetic rats (group III), indicating a hypercoagulable state (P < 0.05). Sildenafil treatment in diabetic rats (group IV) significantly restored aPTT toward normal levels.
Additionally, diabetic rats showed elevated fibrinogen concentrations and markedly decreased levels of anticoagulant proteins C and S compared to controls (P < 0.05). Treatment with SC corrected these abnormalities to a significant extent, lowering fibrinogen and increasing proteins C and S in group IV compared to the untreated diabetic group.
Table 3. Effect of sildenafil on coagulation parameters.
Parameter | Group I (control) | Group II (control + SC) | Group III (diabetic) | Group IV (diabetic + SC) |
PT (sec) | 20.52 ± 0.23 | 19.95 ± 0.27 | 20.55 ± 0.50 | 20.41 ± 0.39 |
aPTT (sec) | 41.46 ± 2.5 | 41.67 ± 2.7 | 37.11 ± 2.1 * | 40.25 ± 2.5 £ ※ |
Fibrinogen (mg/dL) | 230 ± 0.94 | 229.42 ± 1.50 | 298.3 ± 0.96 * | 251.06 ± 0.78 £ ※ |
Protein C (%) | 56.93 ± 0.81 | 56.60 ± 1.23 | 36.82 ± 0.93 * | 55.57 ± 2.07 £ ※ |
Protein S (%) | 62.34 ± 0.92 | 62.42 ± 1.4 | 41.03 ± 0.85 * | 56.01 ± 0.87 £ ※ |
Data expressed as mean ± SE. *: P < 0.05 vs. groups I and II; £: P < 0.05 vs. group III; ※: P < 0.05 vs. groups I and II.
The present study assessed the therapeutic impact of oral sildenafil citrate (20 mg/kg BW) over six weeks on metabolic and hematological disturbances in STZ-induced diabetic rats. STZ, structurally analogous to glucose, selectively damages pancreatic beta cells and effectively induces a hyperglycemic state. Our model successfully maintained elevated blood glucose levels throughout the experiment, consistent with prior literature [9, 10].
The findings revealed that sildenafil significantly improved glycemic markers (glucose, insulin, and HbA1c) and mitigated hepatic and renal dysfunctions often observed in diabetic conditions. Notably, it also restored coagulation balance by reducing fibrinogen levels and increasing anticoagulant proteins C and S.
These outcomes align with prior investigations. Meky et al. reported significant improvements in glucose homeostasis and coagulopathy in diabetic rats following sildenafil administration [11-13]. Similarly, clinical research by Ramirez et al. [7] and Zimmermann et al. [8] found that sildenafil improved insulin sensitivity and glucose metabolism in humans.
Thus, this study adds further evidence to the growing body of literature supporting the potential of SC as an adjunctive treatment for diabetes, especially in modulating vascular and metabolic dysfunctions.
Chronic inhibition of phosphodiesterase-5 (PDE-5) has been linked to enhanced insulin activity in rats [4]. PDE-5 inhibitors are believed to increase insulin sensitivity and thereby improve endothelial function [14]. Endothelial dysfunction is considered a contributing factor to insulin resistance and the development of type 2 diabetes [15], partly by diminishing nitric oxide (NO) availability and reducing cyclic guanosine monophosphate (cGMP) levels [16]. Since cGMP promotes glucose uptake in muscle tissue, preventing its decline may help boost insulin activity. Notably, SC can cross the blood-brain barrier, and PDE-5 is expressed in brain tissue [17], suggesting that central nervous system cGMP signaling may regulate insulin function and energy balance [14].
El Sayed et al. [12] demonstrated that SC administered to diabetic rats in doses of 5–20 mg/kg body weight led to dose-dependent reductions in blood glucose and increases in serum insulin. They proposed that inhibiting cGMP breakdown activates protein kinase G, which phosphorylates hormone-sensitive lipase, thereby promoting free fatty acid release and elevating energy expenditure. These results align with those of Taha et al. [13], who reported a 48% reduction in blood glucose and a 17% increase in insulin levels following SC treatment (10 mg/kg BW) in diabetic rats over 4 weeks. The current study supports the hypothesis that SC mimics insulin’s glucose-lowering effects.
However, these findings contrast with those of Milani et al. [18], who found no significant decrease in blood glucose levels after 15 days of SC treatment at a lower dose (1 mg/kg BW), suggesting dosage plays a critical role. Therefore, further studies should aim to determine the optimal dosage and treatment duration for SC in glycemic control.
Hyperglycemia, which is closely associated with oxidative stress, is a key factor in the development of diabetic complications [2, 19]. Red blood cells are particularly vulnerable because of their role in oxygen transport. Consistent with earlier findings, our study showed elevated glycated hemoglobin levels in diabetic rats. However, daily SC administration for 6 weeks significantly reduced HbA1c levels, indicating a protective effect against oxidative damage. In diabetes, protein glycation affects hemoglobin, other erythrocyte components, and serum proteins. Over time, this leads to the accumulation of advanced glycation end-products (AGEs), which impair protein function and structure, exacerbating complications [20].
The liver, a vital insulin-responsive organ, is central to glucose regulation [21]. Hepatic injury is common in diabetes, characterized by elevated liver enzymes, necrosis, and inflammation [22]. In our study, diabetic rats exhibited significantly higher AST and ALT levels—markers of liver damage—corroborating previous reports [23]. These enzymes typically leak into the bloodstream during liver cell injury. SC has been shown to influence liver hemodynamics [24]. Our findings revealed that SC treatment significantly lowered AST and ALT levels, suggesting hepatoprotective effects. This is consistent with prior studies indicating SC’s role in mitigating hyperglycemia-induced liver damage [25], including histological improvements observed by Hameed and Farooq after 21 days of SC treatment in mice [26].
We also assessed kidney function by measuring serum creatinine and urea levels. Diabetic rats had significantly elevated values, consistent with earlier studies. For instance, Nogueira Júnior et al. [27] observed raised creatinine and urea in diabetic rats administered STZ (45 mg/kg), while Khan and Ola [28] reported similar increases one week after diabetes induction. As in humans, diabetic rats experience renal structural and functional damage [29]. The elevated urea and creatinine in our diabetic group suggest nephron impairment and diabetic kidney disease progression [30]. This deterioration may stem from hyperglycemia-induced free radical production via glucose auto-oxidation, which damages kidney cells. SC treatment significantly improved these parameters, returning values closer to normal, indicating nephroprotective effects.
Supporting this, previous studies have shown that SC lowers elevated creatinine levels in rats following 5/6 nephrectomy over 8 weeks [31], and Rizk et al. [32] reported that SC protected against cisplatin-induced nephrotoxicity by improving renal biomarkers and histology. These findings highlight SC’s potential in improving renal function in diabetic settings—likely by enhancing nephron function rather than solely preventing hyperglycemia-induced injury.
Although prothrombin time (PT), which assesses the extrinsic coagulation pathway, remained unchanged following STZ administration in this study, activated partial thromboplastin time (aPTT)—a measure of the intrinsic pathway—was significantly prolonged in diabetic rats compared to non-diabetic controls. These two parameters are commonly used to assess the risk of thrombosis or bleeding. Similar findings have been reported by Zhao et al. [33], who observed significant differences in aPTT values between diabetic and healthy animals. Moreover, Tripodi et al. [34] identified a correlation between shortened aPTT and an increased risk of venous thromboembolism, suggesting that shortened aPTT values may be a potential risk factor for such conditions.
Additionally, diabetic rats in the present study showed significantly elevated plasma fibrinogen levels relative to controls, consistent with earlier reports in STZ-induced diabetic models [28]. Fibrinogen, a key determinant of blood viscosity and flow characteristics, has been strongly associated with cardiovascular risk, particularly in individuals with type 2 diabetes [35]. Glycation of fibrinogen is thought to promote the formation of denser fibrin clots composed of thinner fibers, which are more resistant to fibrinolysis and thus contribute to a prothrombotic state [35].
Treatment with sildenafil citrate (SC) significantly prolonged aPTT and reduced fibrinogen levels in diabetic rats, while PT remained unaffected. The observed aPTT prolongation may reflect decreased activity or inhibition of coagulation factors involved in the intrinsic and common pathways, particularly factors II, V, and X. SC may mediate this effect by suppressing thrombin generation or by directly or indirectly inhibiting factor Xa and its cofactor Va.
Beyond their anticoagulant roles, proteins C and S also contribute to endothelial barrier stability and inflammatory regulation. However, in diabetic states, these proteins are susceptible to non-enzymatic glycation [36], leading to inactivation and subsequent hypercoagulability. In this study, activity levels of both proteins C and S were significantly lower in diabetic rats than in controls. These deficits likely stem from glycation-induced structural alterations [37]. Notably, SC administration significantly improved the activity of both proteins, suggesting a protective or restorative effect on the natural anticoagulant system.
This study demonstrates that oral administration of SC at 20 mg/kg over 42 days ameliorates a range of biochemical and hematological alterations associated with diabetes in STZ-induced diabetic rats. No adverse effects of SC were observed in non-diabetic controls. In diabetic rats, however, SC administration produced significant improvements, particularly in renal function, as evidenced by normalized serum creatinine and urea levels. While rare instances of SC-associated hepatotoxicity have been reported, the underlying mechanisms remain unclear. In the current study, SC treatment led to improved liver enzyme profiles, suggesting a hepatoprotective effect.
Furthermore, SC contributed to correcting coagulation abnormalities by prolonging aPTT, reducing fibrinogen levels, and restoring protein C and S activity. These findings support the therapeutic potential of SC in mitigating vascular and coagulation-related complications of diabetes. Further research is warranted to explore the clinical implications and underlying mechanisms of these effects.
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